The present invention relates to a film thickness measuring method for detecting a film thickness of a member to be processed with use of an emission spectroscope in such processes as fabrication of semiconductor integrated circuits and a processing method of the member with use of the film thickness measuring method. More particularly, the present invention relates to a film thickness measuring method of members to be processed, preferred so as to measure a film thickness of each layer formed on a substrate in etching processing that employs plasma discharge and obtain a predetermined thickness. The present invention also relates to a processing method of those members with use of the film thickness measuring method.
Dry-etching is one of the main techniques having been employed widely in fabrication processes of semiconductor wafers so as to remove layers formed with various materials thereon. Especially, the dry-etching has been employed to remove dielectric material layers or form patterns on those layers. And, the most important point for controlling process parameters is considered to be decision for endpoints of etching processing so as to stop the etching at each predetermined thickness during the processing.
The light emission intensity of a specific wavelength changes with the progress of the dry-etching processing of semiconductor wafers. One of the conventional etching endpoint detecting methods having been employed for semiconductor wafers, therefore, detects changes of such the light emission intensity of a specific wavelength from plasma during dry-etching processing so as to detect an etching endpoint of a specific film according to this detected emission intensity change. At this time, it is strongly demanded to prevent misdetection of such the endpoint of etching processing, to be caused by irregularity of the detected waveform due to a noise. A well-known method for detecting such the changes of the light emission intensity accurately is disclosed in JP-A-61-53728 and JP-A-63-200533, etc. The moving average method is employed JP-A-61-53728 and the primary least square approximation processing is performed for noise reduction in JP-A-63-200533.
Now that sizes of semiconductors are becoming smaller and the packing density of them is becoming higher, the open area ratio, (area to be etched on a semiconductor wafer) is becoming smaller. And accordingly, the emission intensity of a specific wavelength to be fetched into a light detector from a photo sensor is becoming weaker. As a result, the level of the sampling signal output from the light detector is becoming lower, so that it is becoming difficult for an endpoint determining device to detect endpoints of etching processing accurately according to such the sampling signal output from the light detector.
To detect an endpoint of etching processing so as to stop the etching, it is important that the residual thickness of a dielectric layer should actually become equal to a predetermined value. In the conventional processing, however, all the processes are monitored by a time thickness controlling technique that premises that the etching speed is fixed for all types of layers. An etching speed, for example, is found by processing sample wafers beforehand. According to this method that employs a time monitoring method, therefore, the etching processing stops when a time corresponding to a predetermined etching film thickness is up.
However, an actual film, for example, an SiO2 layer formed by the LPCVD (Low Pressure Chemical Vapor Deposition) method is well known as a layer that is low in reproducibility. The allowable error of film thickness to occur due to a processing fluctuation in the LPCVD is equivalent almost to 10% of the initial thickness of the SiO2 layer. Consequently, the time monitoring method cannot measure the actual final thickness of the SiO2 layer left on the subject silicon substrate. And, final measurement of the actual film thickness is done with use of a standard spectroscopic interferometer. When over-etching is detected, the subject wafer is discarded as an NG one.
It is also well known that an insulation film etching apparatus often causes etching speed-down with time while the etching is repeated. Sometimes, the etching stops on the way. Such the problem must be avoided. In addition, it will also be important to monitor changes of the etching speed with time so as to assure stable etching processing. And, none of the conventional methods has been effective to cope with such the changes and fluctuations of the etching speed with time; the method just monitors the time for determining the end of etching processing. Besides, the decision for the end of etching processing has not been satisfactory when the etching time is as short as about 10 seconds, since the preparing time for the decision, as well as the decision time unit must be as short as possible. Furthermore, an insulation film area to be etched is often less than 1%, so the change of the plasma light emission intensity from a reaction product generated by etching is so small. This is why there has not been practical and reasonable price systems so far, although an etching endpoint decision system that can detect even a slight change of a light emission intensity has been demanded.
On the other hand, there are other well-known methods for detecting endpoints of etching processing on semiconductor wafers. The methods are disclosed in JP-A-5-179467, JP-A-8-274082, JP-A-2000-97648, and JP-A-2000-106356, etc. and each of those methods uses an interferometer. According to those methods that use an interferometer respectively, a monochrome laser beam is exposed at a vertical incidental angle on wafers composed of laminated layers formed with various types of materials. For example, for a wafer consisting of an SiO2 layer and an SiO3N4 layer laminated thereon, interference fringes appear on the wafer due to a light reflected from the top surface of the SiO2 layer and another light reflected from the boundary face between the SiO2 layer and the Si3N4 layer. And, the reflected lights are led into a proper detector, thereby generating a signal whose intensity changes according to the thickness of the SiO2 layer during etching processing. When the top surface of the SiO2 layer is exposed during the etching, both of the etching speed and the etched film thickness can be monitored accurately and continuously. Instead of the laser beam, a predetermined light discharged by plasma may be measured with use of a spectrometer. This is also a well-know method.